The invention relates generally to relay contact protection systems and more particularly to such systems in which the relay is DC-energized and the load is a three-phase motor.
Mechanical wear of electromagnetic relays and contacts thereof is typically minimal. However, electrical contact erosion severely limits the useful life of the relay contacts. But, relay switching without current flow through the contacts eliminates arcing of the contacts and thereby greatly improves the useful life thereof. It is generally accepted by those skilled in the art that relay switching without current flow or voltage potential at the contacts thereof can increase the useful life of such relays by a factor of approximately 10. This operational mode of relay switching in which no current flow or voltage potential exists at the contacts is called "dry switching mode".
Some prior art methods for protecting relay contacts from erosion involve selective dry switching and wet switching i.e., contact switching while current is flowing therethrough. An example of such a prior art method is disclosed in U.S. Pat. No. 4,617,604 to Narimatsu. The Narimatsu method utilizes a circuit which alternates operation of a relay between a dry switching mode and a wet switching mode. The frequency and duration of the operation in the wet switching mode is dependent on the relay characteristics. The wet switching mode is utilized to remove oxidized film and other extraneous substances on the contact faces by the arcing or spark. Operation of the relay in the dry switching mode is utilized to prevent excessive abrading of the relay contacts. However, a primary disadvantage of such prior art methods is that their utilization of wet switching typically produces contact erosion and thereby shortens the relay contact's useful life.
Other prior art systems include a relay protecting circuit connected in parallel to the relay. An example of such a circuit is disclosed in U.S. Pat. No. 3,639,808 to Ritzow. The Ritzow circuit utilizes a triac connected in parallel with the contacts. The triac is controlled either by a secondary winding magnetically coupled to the relay coil or to a transformer connected in parallel to the relay coil. The triac conducts the load current for a short period of time during contact closure in order to prevent contact arcing during the closure. However, triacs are prone to triggering spontaneously when subjected to voltage transients exceeding the triac DV/DT parameter. Consequently, a primarly disadvantage of such circuits is that they are not generally reliable.
Another prior art circuit for protecting relay contacts from erosion synchronizes the operation of the relay with a power supply which characteristically has a known alternating current waveform. An example of such a prior art circuit is disclosed in U.S. Pat. No. 5,055,962 to Peterson. The Peterson circuit controls the actuation of the electromagnetic relay so that the relay contacts open and close at a pre-selected time in a power line waveform. However, such circuitry could not be utilized in a system in which the power line waveform is random and therefore not predictable.
Still other prior art systems for preventing or minimizing arcing of relay contacts utilize circuitry to interrupt the flow of alternating current when the current waveform passes through zero. An example of such a system is disclosed in U.S. Pat. No. 3,223,888 to Koppelmann. The Koppelmann device utilizes two serially connected mechanical switches which have rectifiers connected in parallel therewith. However, a primary disadvantage with such systems is that the time it takes for relay contacts to fully close (and stop bouncing) is generally much longer than the period of time when the waveform is at or proximal to the zero crosspoint. Thus, although such circuits reduce arcing, significant arcing nevertheless still remains and produces erosion of the relay contacts. Moreover, shorting of the diodes used in such circuits will result in a continuous load current supply to the load without a means of stopping the load current flow thereby resulting in undesired operation of the load. Thus, a primary disadvantage of such circuits is that they are susceptable to malfunction.
Other prior art circuits for preventing arcing of relay contacts employ an additional relay to accomplish their desired purpose. An example of such a circuit is disclosed in U.S. Pat. No. 3,046,451 to Kiesel. The Kiesel circuit utilizes a pilot relay in conjunction with the main relay to control the operation of the main relay contacts. A choke provides a time delay ensuring that the time of closure of the main contacts occurs when the alternating current voltage is at or within a few electrical degrees of the zero voltage crosspoint. However, a primary disadvantage with such circuitry is that they must be keyed to a 60-hertz AC source and therefore could not be utilized in a system having a random waveform current source. In addition, such circuitry requires both fine tuning of the adjustable choke and selection of the diode characteristics for accomplishing the desired purpose. Thus, incorporating such circuitry in many systems would be unduly labor-intensive and thus pragmatically too expensive.
Still other prior circuits for protecting relays against contact degradation incorporate circuits connected to an AC signal input which is at a random phase. An example of such a circuit is disclosed in U.S. Pat. No. 4,937,703 to Adams. The Adams circuit utilizes a detector circuit incorporated in the relay driver circuit to receive the AC signal at a random phase. The detector circuit correspondingly provides a detector signal to a threshold responsive circuit, such as a timer, which drives the relay. The detector circuit responds to both the negative as well as the positive cycle of the AC input current and thereby enables the relay contacts to close randomly upon either the positive or negative cycle of the AC signal. Such systems thus prevent transfer of material (going in only one direction) between the contacts which would otherwise significantly degrade the contacts. However, a primary disadvantage with such circuits is that they are only applicable to single phase devices. In addition, such circuits are designed to be used in systems where the relay is AC-energized rather than DC-energized.
What is needed is a relay contact protection system which does not need to be synchronized with the AC load current waveform so that it may be used in systems in which the load current phase is random. In addition, what is needed is such a system which may be used where the relay transfers current to a three-phase device. Moreover, what is also needed is such a system which is reliable and generally fail-safe so that it may be used in aircraft applications.